1. Field of the Invention
The present invention relates to an autofocus mechanism adjusting a focal position with respect to a work piece.
2. Description of Related Art
Conventionally, an autofocus mechanism used in a measurement device is known (see Japanese Patent Laid-open Publication No. H07-105547). The autofocus mechanism uses a method of emitting light at a work piece and identifying a focal position based on light reflected by the work piece. Specifically, the reflected light received by a photodiode is converted to an electric signal. Then, the electric signal is amplified by an amplifier and the focal position is identified based on the amplified signals.
However, in a case where reflectance of the work piece is low, the amplified signals obtained in the autofocus mechanism are small. Thus, when electronic noise is present in the amplified signals, identification of the focal position becomes difficult. Specifically, accuracy of the autofocus is reduced. Moreover, feedback control, which tracks a surface of the work piece based on the focal position, cannot be performed stably.